I think Roberts and Mangum knew each other. They may have known each other by different aliases, but I think they knew each other. Attorney Mark Siemon, who's not the brightest bulb refers to the them as friends in an interview.
Many people took things for granted just because the media forwarded them as truths.
Look at what else they forwarded as fact: condoms, date-rape drugs, broomstick assaults, strangulations, requesting black strippers, non-cooperation of the team, evasiveness by the team, first 911 caller was not involved with Party, it was the woman's first time dancing, she had been beated terribly, she had vaginal and anal injuries, she was a good mother, Kim was looking out for her, etc, etc, etc.
They may even have had a contentious or competitive relationship - but I think they knew each other.
It appears these two women were prostitutes working the same area. From photographs, it also appears both were willing to engage in sex with other woman before audiences. I would think there would be ample opportunity for them to cross paths in the execution of their professional duties.